(1) Field of the Invention
The present invention relates to optical transmission equipment and to an integrated circuit. More particularly, the present invention relates to optical transmission equipment comprising an optical modulation unit including a plurality of optical modulators or an optical demodulation unit for receiving a multilevel modulated optical signal from an external optical transmission line and outputting a plurality of electric high-speed serial digital signal streams converted from the received signal and to an integrated circuit applicable to the optical transmission equipment.
(2) Description of Related Art
The recent increases in the number of wavelengths in WDM transmission and in optical signal modulation speed have increased an amount of information (transmission capacity) that can be transmitted through a single optical fiber. However, there is a feeling that the transmission capacity has reached a limit at about 10 Tbit/s (Terabit/second) and has continued to hover therearound for these several years. This is because the wavelength band usable for optical transmission has reached a limit under constraints on the bandwidth (totaled to about 80 nm=10 THz when the C-band, L-band, and S-band are viewed in combination) of the optical fiber amplifier so that there is no room for an increase in the number of wavelengths in WDM transmission. In such a situation, to increase the optical transmission capacity, it has become essential to improve the use efficiency of the frequency band by devising an improved signal modulation format and fill up the limited frequency band with a larger number of optical signals.
From the 1960s onward, the application of a multilevel modulation technology to wireless communication has enabled high-efficiency signal transmission featuring a spectral efficiency over 10. Multilevel modulation, which is effective in wireless communication, can also be considered as a promising technology even in signal transmission using an optical fiber as a medium so that numerous studies have been made conventionally thereon.
For example, a QPSK (Quadrature Phase Shift Keying) that performs four-level phase modulation is reported in Document 1: R. A. Griffin, et, al., “10 Gb/s Optical Differential Quadrature Phase Shift Key (DQPSK) Transmission using GaAs/AlGaAs Integration,” OFC 2002, Paper PD-FD6, 2003. Further, 16-level amplitude and phase modulation, which is a combination of four-level amplitude modulation and four-level phase modulation, is reported in Document 2: Kenro Sekine, Nobuhiko Kikuchi, Shinya Sasaki, Shigenori Hayase and Chie Hasegawa, “Proposal and Demonstration of 10-Gsymbol/sec16-ary (40 Gbit/s) Optical Modulation/Demodulation Scheme,” Paper We3.4.5, ECOC 2004.
As examples of a modulation format which allows longer-distance optical transmission than prior art by ideally modulating the phase and amplitude of an optical signal simultaneously, there have been proposed optical duo-binary modulation, DPSK (Differential PSK) modulation, and the like.
FIG. 23 shows an example of the basic structure of a conventional optical transmitter 140 for binary intensity modulation.
The optical transmitter for binary intensity modulation comprises a 16-bit-parallel signal input terminal 101 for receiving in parallel sixteen streams of a 3-Gbit/s electric low-speed digital signals in the XAUI format. The electric low-speed parallel digital signals supplied from the parallel input terminal 101 are inputted to a parallel elastic buffer (EB) circuit 102 and outputted to a multiplexing circuit 141 with their respective timings being matched. The multiplexing circuit 141 time-multiplexes 3-Gbit/s×16, i.e., the total of 48-Gbit/s digital data and converts the frame format from XAUI to SONET. The resultant SONET signal is outputted as a 40-Gbit/s high-speed serial digital signal stream from the multiplexing circuit 141 to a transmission line 107.
The high-speed serial digital signal stream is appropriately amplified and supplied to an optical intensity modulator, e.g., a Lithium-Niobate type optical intensity modulator 110 coupled to an optical fiber transmission line. To the optical intensity modulator 110, output light from a semiconductor laser diode (LD) 108 serving as a signal source for an optical fiber is inputted via an optical fiber line 111-1. The output light from the semiconductor laser 108 is subjected to binary ON/OFF intensity modulation by the optical intensity modulator 110 and outputted as output light 113 to an output optical fiber 112 via an optical fiber line 111-2.
FIG. 24 shows a structure of the parallel elastic buffer circuit 102 shown in FIG. 23.
The 3-Gbit/s digital signals d0 to d15 supplied from sixteen signal lines 101 connected to the parallel input terminal 101 are inputted to FIFO (FIRST IN FIRST OUT) circuits 142-1 to 142-16 each having a 10-bit memory capacity. From the FIFO circuits, electric digital signals (d0 to d15) are outputted in parallel to output signal lines with their respective timings being adjusted.
FIG. 25 shows an example of the basic structure of a conventional optical receiver 150 for binary intensity modulation.
A 40-Gbit/s input optical signal 151 in the SONET format supplied from an input optical fiber 152 is inputted to a photo diode 153 via an optical fiber line 111. The input optical signal is converted into an electric digital signal by the photo diode 153 and inputted to a clock extraction and data recovery circuit (CDR) 154 where the digital signal is converted to a high-speed serial digital signal stream. An output signal stream from the CDR 154 is inputted to a demultiplexer (DEMUX) 155 via the transmission line 107 to convert the signal into 3-Gbit/s×16 signals in the XAUI format and outputted as low-speed parallel digital signals to an output transmission path 156.
An integrated circuit (IC) mounting circuitry elements equivalent to 101, 102, and 141 shown in FIG. 23 is proposed, for example, in Document 3: MAX3831/MAX3832+3.3 V, 2.5 Gbps, SDH/SONET, 4-Channel Interconnect Multiplexer/Demultiplexer ICs with Clock Generator available from Maxim Integrated Product, Inc., Document Ref.: 19-1534; Rev 1: October 1999.
In the IC proposed by Document 3, a final output is a 2.4-Gbit/s digital signal and 622-Mbit/s digital signals are inputted to four signal lines equivalent to the parallel input terminal 101. These input signals are converted into a 2.4-Gbit/s high-speed serial digital signal by a 4:1 multiplexing circuit MUX after adjusting their timings by an elastic store circuit corresponding to the elastic buffer circuit 102 and outputted to an output terminal.
The elastic store circuit disclosed in Document 3 has a 10-bit-length memory capacity and has been adjusted to output four low-speed signal data streams with equal timings immediately after the rising edge of are set signal. The elastic store circuit also has the function of absorbing the skew of the low-speed signals such that the timings are automatically maintained provided that a shift in data timing observed thereafter falls within a range of ±7.5 nS (±4.7 bits).
On the other hand, a precoder comprised of a low-speed circuit as an example of the prototype of an IC for optical modulation in an optical duo-binary format is reported, for example, in Document 4: Mikio Yoneyama, Kazushige Yonenaga, Yoshiaki Kisaka, and Yutaka Miyamoto, “Differential Precoder IC Modules for 20- and 40-Gbit/s Optical Duobinary Transmission Systems,” IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, Vol. 47, NO. 12, December 1999.
To increase the optical transmission capacity in a state with a limited number of wavelengths usable in an optical fiber for WDM transmission, it is necessary to prepare plural pairs of electric transmission signal circuits each comprised of, e.g., the parallel elastic buffer (EB) circuit 102 and the multiplexing circuit 141 shown in FIG. 23, supply high-speed serial digital signals outputted from the plurality of transmission signal circuits in parallel to a plurality of optical modulators coupled to an optical transmission line (optical fiber), thereby implement a multilevel optical modulation transmitter for simultaneously modulating the amplitudes and phases of optical signals with a plurality of information signals, and fill the limited frequency band on the optical transmission line with a larger number of optical signals.
For example, the case is assumed where transmission digital signals are subjected to parallel-serial conversion such that the first to N-th bits are inputted in parallel to the first EB circuit and the subsequent (N+1)-th to 2N-th bits are inputted in parallel to the second EB circuit. By matching timings for outputting the individual bits in the first and second EB circuits, it becomes possible to synchronize a first digital signal (the first to N-th bits) supplied from the first multiplexing circuit connected to the first EB circuit to the first optical modulator with a second digital signal (the (N+1)-th to 2N-th bits) supplied from the second multiplexing circuit connected to the second EB circuit to the second optical modulator. What is important here is whether or not a receiver side can correctly regenerate the (N+1)-th to 2N-th bits in continued relation to the first to N-th bits from the optical signals subjected to the multilevel modulation in the first and second optical modulators.
In the case where the first and second optical modulators are connected in a tandem configuration to an internal optical transmission line, e.g., the second digital signal which reaches the output optical fiber immediately after the modulation by the second optical modulator and the first digital signal which reaches the output optical fiber through the second optical modulator after the modulation by the first optical modulator have different path lengths from the multiplexing circuits to the output optical fiber. Accordingly, even though the first and second digital signals are outputted in in-phase relation from the first and second multiplexing circuits, a phase difference between the first and second digital signals occurs in multilevel modulated light observed in the output optical fiber. Therefore, in such optical transmission equipment using the multilevel modulation that multiplexes on the same output optical fiber a plurality of transmission digital signals having passed through different optical modulators, it is necessary to match signal propagation times in the individual digital signal paths including the internal optical transmission line.
Specifically, the multilevel optical modulation transmitter is required to have the function of individually adjusting signal delay time in each of the signal paths such that a transmission digital signal which passes through a shorter signal path is supplied to the optical modulator in delay slightly after another transmission digital signal which passes through a longer signal path is supplied the optical modulator and that the plurality of digital signals having been subjected to the optical modulation reach in in-phase relation to the output optical fiber. Likewise, a multilevel optical modulation receiver, in which a multilevel modulated optical signal received from an external optical transmission line is converted into a plurality of electric high-speed serial digital signal streams and these serial digital signal streams are outputted in parallel to a plurality of signal path each extending toward a decoder, also requires to have the function of individually adjusting signal delay in each of the signal paths.
However, none of Documents 1 to 4 described above has shown a practical solution to the problem associated with delay adjustment in individual signal paths that is encountered when transmission data (digital signal stream) is subjected to multilevel optical modulation using a plurality of optical modulators. In the field of wireless communication to which the multilevel modulation is also applicable, there is no useful solution to the problem associated with delay adjustment in individual signal paths that is encountered in the optical transmission equipment described above.